1. Field of the Invention
The present invention relates to a solid electrolyte membrane for solid polymer fuel cells. More specifically, the present invention relates to a solid electrolyte membrane for proton-conducting, direct methanol fuel cells (referred to as DMFC, hereafter) or hydrogen fuel cells.
2. Description of the Related Art
A solid electrolyte membrane is an important material indispensable for electrochemical elements such as solid polymer fuel cells, temperature sensors, gas sensors and electrochromic devices. Among those uses, solid polymer fuel cells have high expectations as one of the features of new energy technologies in future. When a solid electrolyte membrane is used for a fuel cell, it is often called a proton-conducting membrane, since it plays a role of conducting protons.
Among the solid polymer fuel cells, an electric cell using methanol offers promising prospects as a power source for electric cars, since methanol can be supplied as a liquid fuel in the same way as gasoline. In addition, since it is easy to handle, it has high expectations as an electric cell for electric/electronic devices.
Methanol fuel cells are classified into two types: reforming type cells in which methanol is converted into a gas mixture mainly composed of hydrogen, using a reformer; and DMFC's that directly use methanol without using a reformer. Among them, practical application of DMFCs in electric/electronic portable devices is highly expected, because small and light-weight devices can be realized since no reformer is necessary.
On the other hand, organic polymer materials having sulfonic acid groups, carboxylic acid groups, phosphoric acid groups, or the like, are used for a solid electrolyte membrane that is an important element of a fuel cell. As the organic polymer materials, perfluorosulfonic acid containing polymers such as Nafion (trademark name) membrane of Du Pont and Dow membrane of Dow Chemical are conventionally used.
However, though perfluorosulfonic acid containing polymers described above have excellent proton conductivity, there is a problem of great tendency that methanol, which has a high affinity towards water, permeates from the anode side to the cathode side (methanol cross-over), when they are used for a solid electrolyte membrane of DMFCs. When the cross-over occurs, the supplied fuel (methanol) reacts directly with oxygen at the cathode, making it impossible to draw out the energy of methanol as electric power.
In the case of some proton-conducting membranes such as PBI used in Hydrogen fuel cells that do not involve water, the methanol cross-over has been reported to be small. However, in these polymers that have been doped with strong acids such as phosphoric acid or the like, for example, there is a problem that inorganic dopants leach out from the polymer with methanol/water solution, though the methanol cross-over is small.
Besides the above polymer, sulfonated polyphenylene ethers, polyether ketones, polyimides, polybenzoxazoles, polybenzothiazoles, etc. draw attention as materials with low methanol cross-over (see claims and paragraph 2-4 of Japanese Unexamined Patent Application Publication No. 2002-201269, for example). The major problems of these polymers are that sufficient proton conductivity can not be obtained, since appropriate ion channeling structures are hard to be formed in those materials, and also that they degrade under a strong-acid atmosphere, since they have a lot of CH bonds.